Estimating shock cord strength?

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Feb 3, 2009
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How does one estimate what strength of Kevlar cord/tubular nylon/... to use in a rocket? A 1.5lb rocket flying on at most a G clearly needs less than a 5lb. rocket flying on H/Is, is there a way to estimate the potential load the system will experience? Empirical data gathering gets frustrating (ie:fly the rocket - if the cord breaks use a bigger one next time) while using the biggest cord one can buy gets heavy and expensive. Any ideas?
Length has almost nothing to do with the stress a shock cord sees unless it is too short. For elastic type shock cords, 2x-2.5x rocket length is a good rule of thumb. For inelastic type cords like TN and Kevlar, 3x-5x is a generally good rule of thumb. Also, the faster a rocket is traveling at ejection, the higher the load that the cord and laundry are going to feel. Try and time it right, and not only will you avoid the dreaded zipper, but your recovery system will thank you.
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The OP is asking about "strength" not "length".

tho it is good to have an ample length shockcord, 2 to 3 times
the length of the rocket seems to be the norm.
*heh* Yup, what tensile-strength to use... :) It's something that I've wondered for a long time, I've seen cord that ranges from 30lb all the way to 1500lb.
The OP is asking about "strength" not "length".

tho it is good to have an ample length shockcord, 2 to 3 times
the length of the rocket seems to be the norm.

I concur, my 3" dia 42" long King Kraken has over 10 feet of 9/16" 1500 lb. TN, and this is what we supply with the kits as well. There is a military surplus 1/2" OD green TN on the market that I have successfully used on a couple of builds but I don't know the max load it will take.
As a general rule, I would multiply the weight of the rocket by the amount of G forces expected when the parts snap to the end of the cord. 150 is usually a good number.

There are some things that can be done to reduce the shock experienced.

Using long shock cords can reduce the shock because the separating parts slow down before reaching the end of the cords. This works opposite on shock cord for main deploy on dual deploy systems. If the cord is too long, the chute opens but the rocket continues to fall, increasing the shock when it reaches the end of the cord.

I like to loop my shock cords back on themselves and tape every 3- 5 loops together. By the time I'm done I have a taped bundle of shock cord. It's easy to handle, doesn't tangle as easily, and has to break multiple layers of tape to deploy, which helps reduce the shock on the cord.

Using electronic deployment helps too. It ensures the deployment event occurs at apogee. As long as the flight was vertical, that should be the point with a safe, low air speed.
Wow! What a great R&D topic this would be for a Naram. Measuring the G forces of a rocket at seperation. Sounds like a doable thing.